The invention relates to making fiber plies for fabricating fiber preforms for composite parts that include fiber reinforcement and that present the shape of a body of revolution with a non-developable surface, such as for example the surfaces of rings or of a truncated spheres.
A particular field of application for the invention lies in making fiber reinforcements used in fabricating composite strength members for flexible abutments. Flexible abutments are commonly used in the field of propulsion to form hinges between a nozzle and the body of a thruster. Such abutments are formed by alternating rigid composite strength members, each in the form of a spherical ring, and layers of elastically deformable material such as an elastomer.
A method presently used for forming such composite strength members consists in draping and molding plies of carbon fabric preimpregnated with a resin (e.g. an epoxy resin). More precisely, the method comprises the following steps:                cutting out plies from a preimpregnated fabric of carbon fibers or glass fibers, the plies being in the form of annular sectors of a shape that is approximately that of the development of a truncated cone;        draping plies of preimpregnated fabric in a rosette pattern on a male mold element having a surface in the form of a spherical ring corresponding to the inner surface of the spherical strength member to be made, compacting the plies in a vacuum by means of a membrane, putting a female mold element into place having a surface in the form of a spherical ring corresponding to the outer surface of the strength member to be made; and        polymerizing the assembly under pressure, and unmolding the strength member.        
A rigid part with fiber reinforcement is thus obtained that provides better strength in the face of mechanical stresses.
Nevertheless, the above method is very difficult to implement. Draping fiber plies so as to form the fiber reinforcement is an operation that is manual and that is performed directly on a shaping support, thus preventing any accurate control over the orientation and the quantity of fibers at all points of the reinforcement. Putting the female mold element into place can lead to plies slipping and also to wrinkles being formed.
Consequently, that technique does not provide good reproducibility between parts, which parts can then present mechanical characteristics that differ, in particular in terms of fiber concentration and fiber orientation.
When making a laminated flexible abutment, it is important for all of the strength members of the abutment to present shapes and mechanical characteristics that are similar, in order to ensure that the abutment behaves properly.
Furthermore, the plies are formed using preimpregnated fibers that present less flexibility than do dry fibers, thereby making it much more difficult to shape the plies to match the shape of a non-developable body of revolution.
Naturally, other types of composite parts also exist (e.g. engine casings) that present shapes comprising bodies of revolution with non-developable surfaces and that are made up of fiber reinforcement with fibers that are oriented as a function of mechanical forces so as to increase the mechanical strength of the part. Nevertheless, as with a strength member for a flexible abutment, there is no method available enabling fiber layers to be made repeatedly that present fibers in quantities and orientations that are constant so as to form fiber reinforcements with shapes in the form of bodies of non-developable revolution and with geometrical characteristics that are uniform.
Furthermore, certain types of part also need to be reinforced locally. For this purpose, the fiber reinforcement must present increased thickness of fibers in determined locations. Making fiber layers with thicker fiber portions suitable for deforming and matching the shapes of non-developable bodies of revolution becomes even more difficult, in particular in terms of controlling the orientation of the fibers and in terms of reproducibility between layers.